Display device and control method thereof with brightness and transmittance control

ABSTRACT

Provided is a display device including: a first panel of which light transmittance is controlled; a second panel of which light transmittance is controlled, the second panel being disposed on the rear surface side of the first panel; a light emitting unit of which brightness is individually controlled for each divided area, the light emitting unit being disposed on the rear surface side of the second panel; a second panel control unit configured to control the transmittance of the second panel based on a brightness distribution of the light emitting unit so that the brightness of the transmitted light of the second panel becomes uniform; and a first panel control unit configured to control the transmittance of the first panel based on image data.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a display device and a control methodthereof.

Description of the Related Art

A problem of a transmission type liquid crystal display device, of whichliquid crystal panel is constituted by one panel, is a decrease incontrast caused by leakage of the backlight. One technique to improvethe contrast is to insert a second liquid crystal panel (hereaftercalled “secondary liquid crystal panel”) between a conventional firstliquid crystal panel (hereafter called “primary liquid crystal panel”)and a backlight. Thereby contrast can be improved because a gradationnumber equal to the product of the display gradation numbers of the twoliquid crystal panels can be expressed (Japanese Patent ApplicationLaid-Open No. H5-88197).

Another technique to improve the contrast is dividing a screen into aplurality of areas, and controlling the brightness of the backlight andtransmittance of the liquid crystal panel for each divided area based ona feature value of the image data of each divided area. Thereby blackfloats can be suppressed and contrast can be improved (Japanese PatentApplication Laid-Open No. 2002-99250).

SUMMARY OF THE INVENTION

If the brightness of the backlight is controlled for each divided area,the brightness of the backlight is distributed non-uniformly. Thebrightness of light (output brightness, display brightness) output fromthe liquid crystal panel of the display device is influenced by thebrightness distribution of the backlight. If the brightness distributionof the backlight is not uniform, the brightness of the image data cannotbe accurately reproduced in display. In Japanese Patent ApplicationLaid-Open No. 2002-99250, the influence of an unsmooth brightnessdistribution of the backlight is offset by controlling the transmittanceof the liquid crystal panel in accordance with the brightnessdistribution of the backlight.

To further increase the contrast, the backlight brightness must bereduced. In this case, the backlight brightness distribution decreasesas the backlight brightness decreases. To offset the influence of thebacklight brightness distribution, the transmittance of the liquidcrystal panel must be increased in accordance with the reduction amountof the backlight brightness. In this case, if a display deviceconstituted by one liquid crystal panel is used, the increasedtransmittance may exceed the controllable transmittance of the liquidcrystal panel. Should this occur, the backlight transmitting through theliquid crystal panel cannot be controlled and high gradation cannot beexpressed. As a result, the output image saturates and causesinterference.

It is an object of the present invention to smoothen the backlightbrightness distribution even when the brightness is controlled for eachdivided area, and to further reduce the black brightness in a displaydevice constituted by a liquid crystal panel having a two-layerstructure.

A first aspect of the present invention is a display device having: afirst panel of which light transmittance is controlled; a second panelof which light transmittance is controlled, the second panel beingdisposed on a rear surface side of the first panel; a light emittingunit of which brightness is individually controlled for each dividedarea, the light emitting unit being disposed on a rear surface side ofthe second panel; a second panel control unit configured to control thetransmittance of the second panel based on a brightness distribution ofthe light emitting unit so that the brightness of the transmitted lightof the second panel becomes uniform; and a first panel control unitconfigured to control the transmittance of the first panel based onimage data.

A second aspect of the present invention is a method of controlling adisplay device including: a first panel of which light transmittance iscontrolled; a second panel of which light transmittance is controlled,the second panel being disposed on a rear surface side of the firstpanel; and a light emitting unit of which brightness is individuallycontrolled for each divided area, the light emitting unit being disposedon a rear surface side of the second panel, having: a second panelcontrol step of controlling the transmittance of the second panel basedon a brightness distribution of the light emitting unit, so that thebrightness of the transmitted light of the second panel becomes uniform;and a first panel control step of controlling the transmittance of thefirst panel based on image data.

According to the present invention, the backlight brightnessdistribution can be smoothed when the backlight is controlled for eachdivided area, and the black brightness can be further reduced in thedisplay device constituted by a liquid crystal panel having a two-layerstructure.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a display device of Example 1;

FIG. 2 is a diagram depicting a configuration of the display device ofExample 1;

FIGS. 3A to 3D are a set of diagrams depicting an input image, abacklight configuration, divided areas and pixel values of a segment AB;

FIGS. 4A to 4C are a set of diagrams depicting maximum pixel values,backlight control values and brightness distribution;

FIG. 5 is a flow chart depicting a method of calculating the backlightcontrol value of Example 1;

FIG. 6 is a flow chart depicting a method of calculating a correctedsmoothing gain of Example 1;

FIGS. 7A to 7D are a set of diagrams showing a smoothing gain, acorrected smoothing gain, a secondary liquid crystal brightnessdistribution, and a primary liquid crystal aperture ratio;

FIGS. 8A and 8B show brightness distribution of an output image and theoutput image;

FIG. 9 is a functional block diagram of a display device of Example 2;

FIGS. 10A to 10D depict backlight control values, a brightnessdistribution, a smoothing gain and a corrected smoothing gain;

FIGS. 11A to 11D are a set of diagrams depicting a secondary liquidcrystal brightness distribution, output brightness with/withoutexpansion processing, and a primary liquid crystal aperture ratio; and

FIGS. 12A to 12C show a relationship among a maximum pixel value, abacklight control value, and an expansion ratio, and the output image.

DESCRIPTION OF THE EMBODIMENTS Example 1

FIG. 1 is a block diagram depicting functions of a display device ofExample 1, and FIG. 2 shows the configuration of the display device.

The display device of Example 1 has: a first panel of which lighttransmittance can be controlled; a second panel which is disposed on therear surface side of the first panel and of which light transmittancecan be controlled; and a light emitting unit which is disposed on therear surface side of the second panel and of which brightness can beindividually controlled for each divided area. In Example 1, as shown inFIG. 2, the first panel is a primary liquid crystal unit 107, the secondpanel is a secondary liquid crystal unit 106, and the light emittingunit is a backlight unit 105. The primary liquid crystal unit 107 is adisplay surface. In Example 1, the first panel and the second panel areboth liquid crystal panel examples, but the first panel and the secondpanel are not limited to liquid crystal panels if they are displaypanels of which light transmittance can be controlled for each pixel.For example, a display panel based on a micro electro mechanical system(MEMS) shutter may be used.

The display device of Example 1 is constituted by a feature valueacquisition unit 101, a control value calculation unit 102, a brightnessdistribution calculation unit 103, a smoothing processing unit 104, abacklight unit 105, a secondary liquid crystal unit 106 and a primaryliquid crystal unit 107.

In the configuration in FIG. 1, the feature value acquisition unit 101receives input image data, divides the image area into a plurality ofareas, and acquires a feature value for each divided area. In Example 1,the feature value acquisition unit 101 acquires the maximum value of thepixel values in each divided area (hereafter called “maximum pixelvalue”) as the feature value of each divided area. The maximum pixelvalue is transmitted to the control value calculation unit 102.

The control value calculation unit 102 receives the maximum pixel valuein each divided area acquired by the feature value acquisition unit 101,and calculates the backlight control value (brightness control value) ofeach divided area in accordance with the maximum pixel value. Thecalculated backlight control value is transmitted to the brightnessdistribution calculation unit 103 and the backlight unit 105.

The brightness distribution calculation unit 103 receives the backlightcontrol value of each divided area, which is calculated by the controlvalue calculation unit 102, and calculates the brightness distributionof the light output from the backlight unit 105, based on the backlightcontrol value of each divided area. The calculated brightnessdistribution information is transmitted to the smoothing processing unit104.

The smoothing processing unit 104 receives the input image data and thebrightness distribution information calculated by the brightnessdistribution calculation unit 103. The smoothing processing unit 104executes the processing to smooth the backlight brightness distributionbased on the received input image data and the brightness distributioninformation, and calculates the aperture ratio (transmittance) for eachpixel of the secondary liquid crystal unit 106. The calculated apertureratio information on the secondary liquid crystal unit 106 istransmitted to the secondary liquid crystal unit 106.

The backlight unit 105 receives the backlight control value calculatedfor each divided area by the control value calculation unit 102, andcontrols light emission of the light source of each divided area basedon the received backlight control value.

The secondary liquid crystal unit 106 receives the aperture ratio ofeach pixel calculated by the smoothing processing unit 104, and performsthe second panel control to control the liquid crystal element of eachpixel based on the received aperture ratio.

The primary liquid crystal unit 107 receives the input image data, andperforms the first panel control to control the liquid crystal elementof each pixel based on the received image data.

The processing content of the display device according to Example 1 willbe described using monochrome 8-bit input image data shown in FIG. 3A asan example. In Example 1, the primary liquid crystal unit 107 and thesecondary liquid crystal unit 106 are monochrome liquid crystal panelshaving a same pixel size, and the backlight unit 105 is a whitebacklight. The image area (pixel size) of the input image data is 2048pixels horizontally×1536 pixels vertically. The pixel values of theinput image data are values in the 0 to 255 range, and are controlledsuch that the brightness of the output image (display brightness) of theprimary liquid crystal unit 107 becomes 200 cd/m² when the pixel valueis 255. The gamma value is 1.0. The contrast ratio of the primary liquidcrystal unit 107 and that of the secondary liquid crystal unit 106 areboth 100:1. These specifications are an example, and the display deviceto which the present invention is applied is not limited to theexemplified specifications. The following description should be read, asappropriate, according to the specifications of the display device towhich the present invention is applied.

In Example 1, the backlight unit 105 is constituted by three verticallydivided areas×four horizontally divided areas as shown in FIG. 3B, andthe brightness of the backlight unit 105 can be individually controlledfor each divided area. The feature value acquisition unit 101 acquiresthe feature value of the divided area of the image data corresponding toeach divided area of the backlight unit 105. FIG. 3C shows the dividedareas for which feature values are acquired in the input image data. Inthe following description, to simplify description, one-dimensionalimage data on segment AB in FIG. 3C, out of the image data, will bedescribed as an example. FIG. 3D shows a pixel value distribution of theone-dimensional image data on segment AB in FIG. 3C. FIG. 4A shows theresult of the feature value (maximum pixel value) of each divided areaoutput by the feature value acquisition unit 101.

An example of a calculation method of the backlight control valueaccording to Example 1 will be described. In Example 1, the controlvalue calculation unit 102 determines the backlight control value ofeach divided area as one of two types, depending on whether the maximumpixel value is zero or not. The backlight control value and the outputbrightness value are correlated. For convenience, in Example 1 it isassumed that the backlight control value is set in two levels: 0 or 200.Now the procedure of the calculation method of the backlight controlvalue will be described with reference to the flow chart in FIG. 5. Instep S801, the control value calculation unit 102 determines whether themaximum pixel value of the image data corresponding to the targetdivided area, for which the backlight control value is calculated, iszero or not. The control value calculation unit 102 sets the backlightcontrol value to zero if the maximum pixel value is zero in step S802.And if the maximum pixel value is not zero, the control valuecalculation unit 102 sets the backlight control value to 200 in stepS803. FIG. 4B shows the backlight control value of each divided areaacquired by this processing.

An example of a calculation method of brightness distribution of lightemitted by the backlight unit 105 before transmitting through thesecondary liquid crystal unit 106, according to Example 1, will bedescribed. In Example 1, it is assumed that the light source of eachdivided area of the backlight unit 105 is located at the center of eachdivided area, as shown in FIG. 3B. Each light source emits light inaccordance with the backlight control value that is set in each dividedarea. The brightness distribution of the light from the light source ofeach divided area is assumed to conform to Gaussian distribution. Thebrightness distribution of the light in the entire area of the backlightunit 105 is assumed to be a total of the brightness distribution of eachdivided area according to the superposition principle of light.Therefore in the one-dimensional area along segment AB, the brightnessdistribution of each divided area and the brightness distribution of theentire area of the backlight unit 105 become graph 1001 and graph 1002in FIG. 4C respectively. If the brightness distribution in the dividedarea after transmitting through the secondary liquid crystal unit 106 issmoothed to 200 cd/m², then processing becomes easier since correctionof the input image data for smoothing the brightness distribution isunnecessary. The smoothed brightness distribution, after transmittingthrough the secondary liquid crystal unit 106 that is targeted, is abrightness distribution where the brightness in each divided areabecomes uniform at a value in accordance with the backlight controlvalue shown in FIG. 4B. The target brightness distribution is thebrightness distribution indicated by graph 1003 in FIG. 4C. In FIG. 4C,the peak value of the brightness distribution in each divided areaexceeds the target brightness distribution, which is 200 cd/m². This isallowable because brightness insufficiency at the edge of the dividedarea is prevented in the backlight brightness distribution in the entirearea.

The smoothing processing for the backlight brightness distribution bythe smoothing processing unit 104 of Example 1 will be described. Thesmoothing processing unit 104 calculates a smoothing gain which is setfor the secondary liquid crystal unit 106, so that the backlightbrightness distribution before transmitting through the secondary liquidcrystal unit 106 indicated by graph 1002 in FIG. 4C is smoothed to thebrightness distribution indicated by graph 1003 in FIG. 4C aftertransmitting through the secondary liquid crystal unit 106. By settingthe smoothing gain calculated like this for the secondary liquid crystalunit 106, the brightness distribution after transmitting through thesecondary liquid crystal unit 106 becomes uniform in all divided areasat 200 cd/m², as indicated by graph 1003 in FIG. 4C. However if thissmoothing gain is simply set for the secondary liquid crystal unit 106,the brightness of black pixels (hereafter called “black brightness”) inthe output image becomes 2 cd/m², since the contrast ratio of theprimary liquid crystal unit 107 is 100:1. In this case, the blackbrightness of the image output by the display device is determined onlyby the contrast ratio of the primary liquid crystal unit 107. To furtherreduce the black brightness, the smoothing processing unit 104 sets theaperture ratio (transmittance) of each pixel of the secondary liquidcrystal unit 106 corresponding to the black pixel in the input imagedata to the minimum aperture ratio (transmittance) that can be set forthe secondary liquid crystal unit 106. According to Example 1, in thesmoothing gain calculated as described above, the smoothing processingunit 104 corrects the smoothing gain, which is set for each pixelcorresponding to the black pixel in the input image data, to zero(corrected smoothing gain). If the corrected smoothing gain is set forthe secondary liquid crystal unit 106, the black brightness of theoutput image becomes 0.02 cd/m², based on the product of the contrastratio of the secondary liquid crystal unit 106 and that of the primaryliquid crystal unit 107, and the black brightness can be furtherreduced. In this way, according to Example 1, the smoothing processingunit 104 determines the aperture ratio (transmittance) of the secondaryliquid crystal unit 106 based on the brightness distribution of theentire area of the backlight unit calculated by the brightnessdistribution calculation unit 103 and the input image data.

The procedure of the processing to calculate the corrected smoothinggain will be described in detail with reference to the flow chart inFIG. 6. Here the one-dimensional position coordinate along segment AB ofthe secondary liquid crystal unit 106 is assumed to be Y.

In step S1101, the smoothing processing unit 104 acquires the brightnessdistribution LReal (Y) of the entire area of the backlight unit 105 fromthe brightness distribution calculation unit 103. The backlightbrightness distribution of the entire area is the total of thebrightness distribution of each divided area, as mentioned above, and isindicated by graph 1002 in FIG. 4C.

In step S1102, the smoothing processing unit 104 acquires the targetbrightness distribution LIdeal (Y) after transmitting through thesecondary liquid crystal unit 106. Here as mentioned above, the targetbrightness distribution is set to 200 cd/m² in all of the divided areasin accordance with the backlight brightness value, and is indicated bygraph 1003 in FIG. 4C.

In step S1103, the smoothing processing unit 104 calculates thesmoothing gain Gain (Y) from the backlight brightness distribution ofthe entire area and the target brightness distribution. The smoothinggain Gain (Y) can be calculated by Expression 101.Gain (Y)=LIdeal (Y)/LReal (Y)  Expression 101

Graph 1201 in FIG. 7A shows the distribution of the gain calculated byExpression 101. In this way, the smoothing processing unit 104calculates the smoothing gain of each divided area of the secondaryliquid crystal unit 106 based on the value obtained by dividing thebrightness distribution of the entire area of the backlight unit, whichis calculated by the brightness distribution calculation unit 103, bythe backlight control value of each divided area determined by thecontrol value calculation unit 102.

In step S1104, the smoothing processing unit 104 determines whether thetarget pixel for which the smoothing gain is calculated is a black pixelin the input image data, and if it is a black pixel, the smoothingprocessing unit 104 performs correction in step S1105 so that thesmoothing gain of this pixel becomes zero. According to this procedure,the smoothing processing unit 104 calculates the corrected smoothinggain. The graph 1301 in FIG. 7B shows the corrected smoothing gain,which is calculated when the input image data shown in FIG. 3D is input.

According to this procedure, the brightness distribution aftertransmitting through the secondary liquid crystal unit 106 becomes 2cd/m² in a position of a black pixel, and is 200 cd/m² in a pixel otherthan black in the input image data, as indicated by graph 1401 in FIG.7C. This brightness distribution after transmitting the secondary liquidcrystal unit 106 has been processed to smooth the backlight brightnessdistribution generated by the divided area control, hence no correctionprocessing is necessary to smooth the image data to be input to theprimary liquid crystal unit 107. Furthermore, the brightnessdistribution after transmitting the secondary liquid crystal unit 106has been processed to reduce the black brightness, hence the blackbrightness of the output image of the primary liquid crystal unit 107 isfurther reduced, and higher contrast can be implemented. Even if thebacklight brightness of the black image is reduced to further enhancecontrast, correction processing for smoothing is not performed for theprimary liquid crystal unit 107, therefore gradation is not saturated inthe output image of the primary liquid crystal unit 107.

The effect of Example 1 will be described. In Example 1, the correctionprocessing to smooth the backlight brightness distribution for theaperture ratio of the primary liquid crystal unit 107 is unnecessary.Therefore if the aperture ratio of the primary liquid crystal unit 107is 100% when the input pixel value shown in FIG. 3D is 255, and is 0%when the input pixel value is 0, then the aperture ratio distribution ofthe primary liquid crystal unit 107 becomes as shown in FIG. 7D. Theoutput brightness distribution of the primary liquid crystal unit 107becomes the brightness distribution shown in FIG. 8A, based on thebrightness distribution after transmitting the secondary liquid crystalunit 106 in FIG. 7C and the aperture ratio distribution of the primaryliquid crystal unit 107 in FIG. 7D. This description concerns theprocessing on one-dimensional image data along segment AB as an example,but if a similar processing is executed on the entire image data, theoutput image (display image) of the primary liquid crystal unit 107,when the image data shown in FIG. 3A is input, becomes an image shown inFIG. 8B. As shown in FIG. 8B, the brightness of the input image data canbe displayed with high reproducibility, and the black brightness isfurther reduced.

According to the configuration of Example 1, the display device isconstituted by the liquid crystal panel having a two-layer structure anda backlight that performs divided area control, where the secondaryliquid crystal unit 106 executes the smoothing processing for thebrightness distribution of the backlight. This makes the imageprocessing of the primary liquid crystal unit 107 easier. Further, apixel of which pixel value in the input image data is zero becomes zerotransmittance in both the secondary liquid crystal unit 106 and theprimary liquid crystal unit 107, which allows implementing furtherreduction of the black brightness.

In Example 1, the case when the input image is a monochrome image wasdescribed as an example, but the input image may be a color image. Inthis case, it is preferable that a color liquid crystal panel is usedfor the liquid crystal panel. The brightness distribution of thebacklight is smoothed by the secondary liquid crystal unit 106, hence ifthe white light source is disposed in each divided area of thebacklight, it is preferable that a monochrome liquid crystal panel isused for the secondary liquid crystal unit 106. If a light source groupconstituted by light sources of a plurality of colors (e.g. RGB lightsource group) is disposed in each divided area of the backlight, it ispreferable to use a color liquid crystal panel corresponding to theplurality of colors of the light source group for the secondary liquidcrystal unit 106. In this case, it is preferable to control thetransmittance of each color of the secondary liquid crystal unit 106based on the brightness distribution of each color of the backlight, sothat the brightness of the transmitted light of each color of thesecondary liquid crystal unit 106 becomes uniform within each dividedarea. In Example 1, the number of the area divisions is 4×3 (=12), butthe number of divisions is not limited to this.

Example 2

According to the calculation method of the backlight control value inExample 1, the backlight control value becomes 0 if the maximum pixelvalue acquired by the feature value acquisition unit 101 is zero in thetarget divided area for which the backlight control value is calculated,and becomes 200 if the maximum pixel value is a value other than zero.In Example 2, a case of controlling the backlight control value inmultiple levels, in accordance with the feature value (e.g. maximumpixel value) of the divided area acquired by the feature valueacquisition unit 101, will be described. Since the backlight controlvalue is finely controlled in accordance with the feature value of thedivided area, the black brightness can be further reduced and more powercan be conserved compared with Example 1. In Example 2, a processingprocedure, in the case when the present invention is applied to adisplay device which controls the backlight control value in multiplelevels in accordance with the feature value of the divided area, will bedescribed.

FIG. 9 shows a configuration of the display device of Example 2. Thedisplay device in FIG. 9 has the configuration of the display device inFIG. 1, to which an expansion rate calculation unit 108 and a correctionunit 109 are added. The added-configuration will be described here.

The expansion rate calculation unit 108 receives the backlight controlvalue of each divided area calculated by the control value calculationunit 102. The expansion rate calculation unit 108 calculates theexpansion rate to execute the expansion processing on the input imagedata based on the backlight control value of each divided area. Thecalculated expansion rate is transmitted to the correction unit 109.

The correction unit 109 receives the expansion rate calculated by theexpansion rate calculation unit 108 and the input image data. Thecorrection unit 109 executes the expansion processing on the input imagedata based on the expansion rate. The expanded image data is transmittedto the primary liquid crystal unit 107.

The processing content of the display device of Example 2 will bedescribed using the monochrome 8-bit input image data shown in FIG. 3Aas an example, just like Example 1.

An example of the calculation method of the backlight control value willnow be shown. In Example 2, the control value calculation unit 102determines the backlight control value of each divided area in multiplelevels in accordance with the maximum pixel value of each divided area.The control value calculation unit 102 determines the backlight controlvalue of each divided area as shown in FIG. 10A based on thecorrespondence of the maximum pixel value indicated by the graph in FIG.12A and the backlight control value, and the maximum pixel value of eachdivided area shown in FIG. 4A.

Example 2 shows an example of a calculation method of the backlightbrightness distribution before transmitting through the secondary liquidcrystal unit 106. Based on the same concept as Example 1, in aone-dimensional area along segment AB, the brightness distribution ineach divided area and the brightness distribution in the entire area ofthe backlight unit 105 become like graph 2101 and graph 2102 in FIG. 10Brespectively. The target brightness distribution after transmittingthrough the secondary liquid crystal unit 106 is a brightnessdistribution where the brightness in each divided area is uniform at avalue in accordance with the backlight control value shown in FIG. 10A.The target brightness distribution becomes the brightness distributionindicated by graph 2103 in FIG. 10B.

An example of the smoothing processing for the backlight brightnessdistribution of Example 2 will be described. The smoothing processingunit 104 calculates a smoothing gain which is set for the secondaryliquid crystal unit 106 from the backlight brightness distribution ofthe entire area and the target brightness distribution based on the sameconcept as Example 1. The smoothing gain becomes the distributionindicated by graph 2201 in FIG. 10C. The smoothing processing unit 104also calculates the corrected smoothing gain just like Example 1. Thecorrected smoothing gain becomes the distribution indicated by graph2301 in FIG. 10D.

In the expansion rate calculation processing according to Example 2, theblack brightness reduction effect in Example 2 and the necessity ofexpansion rate calculation processing corresponding to the reduced blackbrightness will be described first.

After the processing procedure described thus far, the brightnessdistribution after transmitting through the secondary liquid crystalunit 106 becomes as indicated by graph 2401 in FIG. 11A. In FIG. 11A, ina divided area of which backlight control value is 200, the brightnessafter transmitting through the secondary liquid crystal unit 106 becomes2 cd/m² at a position of a black pixel in the input image data, andbecomes 200 cd/m² in positions other than black. In a divided area ofwhich backlight control value is 100, on the other hand, the brightnessafter transmitting through the secondary liquid crystal unit 106 becomes1 cd/m² at a position of a black pixel in the input image data, andbecomes 100 cd/m² in positions other than black. In the brightnessdistribution after transmitting through the secondary liquid crystalunit 106, the brightness of a black pixel is further reduced in adivided area of which the backlight control value is low, and thebrightness required for the primary liquid crystal unit 107 can beprovided for pixels other than black pixels.

The expansion processing will now be described. If the aperture ratio ofthe primary liquid crystal unit 107 is set to values shown in FIG. 7Dwithout performing the expansion processing on the input image data, theoutput brightness becomes like graph 2501 in FIG. 11B. As shown in FIG.11B, the brightness of a pixel of which pixel value is 128 in the inputimage data becomes 50 cd/m², which is ¼ the brightness of a pixel ofwhich pixel value is 255, and reproducibility of the brightness of theinput image data is low. To prevent this, the correction unit 109performs the expansion processing on the input image data correspondingto the reduction degree of the backlight control value which is inaccordance with the feature value of each divided area. Thereby theoutput brightness becomes a brightness corresponding to the input imagedata.

An example of the calculation processing of the expansion rate accordingto Example 2 will be described. The reference value of the backlightcontrol value is set to the maximum value (200 in this example) of thebrightness control value calculated by the control value calculationunit 102. The reference value of the backlight control value is abacklight control value determined by the control value calculation unit102 if the maximum pixel value is the upper limit value (255 in thisexample) of the pixel value. Here the backlight control value is 200,which is based on a value assumed to be an output brightness value (200cd/m² in this example) when the maximum pixel value is the upper limitvalue of the image data. The correction unit 109 calculates theexpansion rate such that the expansion rate becomes ×1 when the pixelvalue of the input image data is the upper limit value. Thereby thepixel values of the input image data after the expansion processing doesnot exceed the upper limit value. If the backlight control value of eachdivided area is BL and the expansion rate of each divided area is α,then expansion rate a of each divided area is calculated by Expression201.α=200/BL (where BL≥200/255)α=255 (where BL<200/255)  Expression 201

Here the expansion rate is a uniform predetermined value (255 in thisexample) for the divided areas of which backlight control valuedetermined by the control value calculation unit 102 is smaller than thethreshold (200/255 in this example). This is to prevent the expansionrate from exceeding 255. FIG. 12B shows the relationship between theexpansion rate and the backlight control value.

The aperture ratio of the primary liquid crystal unit 107, based on theinput image data corrected with the expansion rate calculated by theabove mentioned processing procedure, becomes like graph 2710 in FIG.11C, and the brightness distribution of the output image becomes likegraph 2801 in FIG. 11D. The output image becomes like FIG. 12C, wherethe brightness of the input image data is displayed with highreproducibility, and further reduction of black brightness (suppressionof black floats) is implemented.

According to Example 2, the secondary liquid crystal unit 106 performsthe smoothing processing of the backlight brightness distribution whenthe backlight control value is finely controlled in accordance with thefeature value of each divided area, and the input image data iscorrected with the expansion rate in accordance with the reductiondegree of the backlight control value. Thereby the smoothing of thebacklight brightness distribution and the reduction of the blackbrightness can be implemented in a display device constituted by aliquid crystal panel having a two-layer structure and a backlight.

Other Embodiments

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-162831, filed on Aug. 8, 2014, and Japanese Patent Application No.2015-097259, filed on May 12, 2015, which are hereby incorporated byreference herein in their entirety.

What is claimed is:
 1. A display device comprising: a first panel for displaying an image by transmitting light based on image data; a second panel having a plurality of pixels transmitting light forward to the first panel, the second panel being disposed on a rear surface side of the first panel; a light emitting unit emitting light to the second panel, the light emitting unit being disposed on a rear surface side of the second panel and having a plurality of emission control areas, an emission brightness of each of the plurality of emission control areas being individually controllable; one or more processors; and a memory in communication with the one or more processors, the memory storing instructions which, when executed by the one or more processors, cause the one or more processors to: (1) control the emission brightness of each of the plurality of emission control areas of the light emitting unit based on a target emission brightness corresponding to a feature value of a respective divided area of the image data corresponding to each of the plurality of emission control areas; and (2) calculate an irradiation brightness of light irradiating each pixel of the second panel from the light emitting unit in a case where the emission brightness of each of the plurality of emission control areas is controlled based on its corresponding target emission brightness, and (3) control transmittance of the first panel and the second panel, wherein transmittance of a first pixel of the second panel whose corresponding pixel of the image data is a black pixel is controlled to a predetermined transmittance, and wherein transmittance of a second pixel of the second panel whose corresponding pixel of the image data is not a black pixel is controlled based on a value obtained by dividing an emission brightness of an emission control area corresponding to the second pixel by a calculated irradiation brightness of the light irradiating the second pixel of the second panel from the light emitting unit.
 2. The display device according to claim 1, wherein the predetermined transmittance is a minimum transmittance for the second panel.
 3. The display device according to claim 1, wherein the emission brightness of each of the plurality of emission control areas of the light emitting unit is controlled in accordance with pixel values of image data corresponding to each of the plurality of emission control areas.
 4. The display device according to claim 1, wherein expansion processing is performed on the image data based on the target emission brightness of each of the plurality of emission control areas, and wherein the transmittance of the first panel is controlled based on expanded image data obtained by the expansion processing on the image data.
 5. The display device according to claim 1, wherein a first target emission brightness of each of the plurality of emission control areas is determined in accordance with a maximum value of pixel values of image data corresponding to each of the plurality of emission control areas, and wherein an expansion rate, based on which expansion processing is performed on image data corresponding to each of the plurality of emission control areas, is determined based on a value obtained by dividing a reference value, which is a second target emission brightness determined in a case where the maximum value of the pixel values is an upper limit value of the pixel values, by the first target emission brightness of that emission control area.
 6. The display device according to claim 5, wherein an expansion rate of image data corresponding to an emission control area, of which area the brightness control value is smaller than a threshold, is set to a predetermined value.
 7. The display device according to claim 1, wherein the first panel is a liquid crystal panel.
 8. The display device according to claim 1, wherein the second panel is a liquid crystal panel.
 9. The display device according to claim 1, wherein a white light source is disposed in each of the plurality of emission control areas of the light emitting unit, and wherein the second panel is a monochrome liquid crystal panel.
 10. A method of controlling a display device including (a) displaying an image, via a first panel, by transmitting light based on image data; (b) transmitting light, via a second panel, forward to the first panel, the second panel having a plurality of pixels and being disposed on a rear surface side of the first panel; and (c) emitting light, via a light emitting unit, to the second panel, the light emitting unit being disposed on a rear surface side of the second panel and having a plurality of emission control areas, an emission brightness of each of the plurality of emission control areas being individually controllable, the method comprising the steps of: controlling the emission brightness of each of the plurality of emission control areas of the light emitting unit based on a target emission brightness corresponding to a feature value of each divided area of the image data corresponding to each of the plurality of emission control areas; and calculating an irradiation brightness of light irradiating each pixel of the second panel from the light emitting unit in a case where the emission brightness of each of the plurality of emission control areas is controlled based on its corresponding target emission brightness, wherein the transmittance of a first pixel of the second panel, whose corresponding pixel of the image data is a black pixel, is controlled to a predetermined transmittance, and wherein transmittance of a second pixel of the second panel, whose corresponding pixel of the image data is not a black pixel, is controlled based on a value obtained by dividing an emission brightness of an emission control area corresponding to the second pixel by a calculated irradiation brightness of the light irradiating the second pixel of the second panel from the light emitting unit. 